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I have an older Trane furnace. I have had HVAC people out here, but they are not listening to me when I describe what it does, so I was hoping to get some public input on this issue.

The house will heat up to 70, the heat goes out (as it should), but the blower keeps going. No, this is not just a few minutes. If you leave it, the blower continues to run, and the heat never kicks back on. The only way to get it to stop is to turn off the power to the furnace.

The thermostat's fan is is set on "auto" not "on".

We changed the thermostat thinking that was the issue, but that didn't fix it.

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    Does it consistently exhibit this behavior? How long has it been behaving like this? I think it would be trivial to replicate this behavior in front of an HVAC tech. However, my guess is that they're simply not interested in dealing with old equipment. Unless you say "I need a new furnace" then they'll continue to brush you off. If you want to DIY this then you will have to get familiar with the electrical schematics of your furnace. It sounds like something went faulty.
    – MonkeyZeus
    Jan 28, 2022 at 18:09
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    Specifying the exact model of Trane furnace would help us help you.
    – FreeMan
    Jan 28, 2022 at 18:36
  • there could be a jumper on the furnace that sets this as a feature, not a bug.
    – dandavis
    Jan 29, 2022 at 8:55

1 Answer 1

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The furnace control board will run the blower for a period of time after the heating cycle ends in order to cool the exchanger and push the latent heat into the space. The time it runs is usually configurable via jumpers or DIP switches depending on the model.

If it is going continuously and never stopping, there are a couple of possibilities (assuming that as you stated, the thermostat is not calling for constant fan):

  1. It might have a continuous blower setting on the board that is enabled - you would have to check the jumpers/DIP switches and verify.

  2. There might be a problem in the thermostat wiring that is shorting the +24VAC (R) and fan terminals (G) inadvertantly.

  3. The furnace board might just be going bad.

If we had a picture of the control board it might help troubleshooting.

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    could be a thermal sensor gone bad, which makes the furnace think it's hot enough to need the cool-down fan.
    – dandavis
    Jan 29, 2022 at 8:54

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